At present, in WAN (Wide Area Network) services provided by communication carriers, to improve user reliability on communication, 1+N and 1:N redundant configurations for making path in communication carrier networks redundant and switching lines when failures occur to protect user communication are widely adopted. However, to configure redundant paths with these systems, an increase in network cost due to an increase in the number of links poses a problem. Therefore, a ring redundant communication path that can be made redundant with a small number of links is actively adopted.
However, in the ring redundant communication path, when a multi-point to multi-point communication path such as a wide area Ethernet (registered trademark) is provided, ring control for logically releasing a loop and switching a path when a failure occurs is necessary.
Therefore, in spanning tree control methods such as an STP (Spanning Tree Protocol) disclosed in Non-Patent Document 1 and an RSTP (Rapid Spanning Tree Protocol) disclosed in Non-Patent Document 2, a loop configuration is logically prevented by setting a single port on a ring redundant communication path in a blocking condition.
However, in the spanning tree control methods, many kinds of parameters needs to be treated for a path calculation for determining a network configuration. Therefore, there is a problem in that path recalculation cost for path switching during a failure increases and it takes time to switch a path.
Therefore, an EAPS (Ethernet (registered trademark) Automatic Protection Switch) by a ring control system disclosed in Non-Patent Documents 3 and 4 and a switching node control method disclosed in Non-Patent Document 5, a main transfer apparatus on a ring redundant communication path closes one control port to logically prevent a loop, transmits an inspection frame that circulates through the ring redundant communication path, detects a failure when the main transfer apparatus fails to receive the inspection frame, and opens the closed control port to perform path switching to thereby realize quick path switching.
Non-Patent Document 1: IEEE 802.1D-1998 Edition MAC bridges (Chapter 8)
Non-Patent Document 2: IEEE 802.1D-2004 Edition MAC bridges (Chapter 17)
Non-Patent Document 3: RFC3619 Extreme Networks' Ethernet (registered trademark) Automatic Protection Switching (EAPS) Version 1
Non-Patent Document 4: Masato Ando, “LAN Switch Technology—Redundancy Method and Latest Technology—”, pp. 7 to pp. 9, Internet week 2003, [retrieved Mar. 16, 2007], Internet http://www.soi.wide.ad.jp/class/20030038/slides/40/index—1. html
Non-Patent Document 5: Masato Ando, “LAN Switch Technology—Redundancy Method and Latest Technology—”, pp. 16 to pp. 19, Internet week 2003, [retrieved Mar. 16, 2007], Internet http://www.soi.wide.ad.jp/class/20030038/slides/40/index—1.html